The present invention relates generally to printed circuit board assembly processes. More particularly, the present invention relates to a method for isolating a pin of a ball grid array device mounted on a printed circuit board and routing the signal carried by the isolated pin to a new location by attaching a rework wire to the isolated pin.
There has been a steady increase in the densities of circuit assemblies on printed circuits boards (PCB). This increase has been fueled by an ever increasing demand for faster circuit speeds which require minimum distances between circuit packages, increased complexity of functional circuitry, and miniaturization of circuit devices with higher pin counts. Increased densities of PCB assemblies have in turn made it more difficult to perform PCB rework tasks which are frequently required to incorporate design changes, or to correct faults detected during testing or to increase the reliability of PCBs. The increased complexity of performing rework tasks has in turn increased both the total cost and time required to fabricate PCBs. Thus, the efficiency and effectiveness of a rework process directly affects the process yield and reliability of the PCB assembly and fabrication process.
Conventionally, rework processes involving pin isolation and signal routing, are performed entirely on the PCB itself. This is usually accomplished by performing a trace cut on the PCB and attaching a rework wire or an engineering change (E/C) wire to the trace cut or rework bond pad on the PCB. The rework wire is then used to route the signal from the integrated circuit to an alternate PCB location. This method, however, is not feasible for newer devices, such as Ball Grid Array (BGA) ASIC devices, or other BGA devices in which the spacing between the device pins is reduced so much as to prevent the use of rework bond pads. The only alternative in such situations is to perform a trace cut on the PCB, solder the rework wire directly to the trace on the PCB and then route the wire to the new location.
The phrase "BGA device" as used in this specification refers to a BGA package with the IC housed in the package. The word "pin" as used in the BGA device context refers to a device pad on the BGA package which has a ball of solder attached to it.
The flow chart in FIG. 1 depicts a conventional method for isolating BGA device pins and routing the pin signals to alternate locations. As shown in FIG. 1, at step 2, a trace cut is made on the PCB at the BGA site. At step 4, the solder layer covering the trace is peeled off to allow soldering of a rework wire to the trace. Next, at step 6, a rework solder wire is attached to the BGA trace close to the pad on the PCB using a high temperature solder. The rework wire is not allowed to touch the pad because the signal which is being carried by the PCB device requires the solder ball to be in contact with the pad. The rework wire is soldered to the trace using a high temperature solder which will not reflow when the BGA device reflows.
At step 8, the rework wire is routed on the PCB between the BGA pads. At step 10, the rework wire may optionally be secured by attaching it to the PCB. The BGA device is then screen printed at step 12. At step 14, the BGA device is placed in a BGA reflow system and reflowed. Finally, at step 16, the rework wire is used to route the signal at the repaired location on the PCB to an alternate location on the PCB.
FIG. 2 depicts a PCB 26 illustrating the results obtained by using the prior art method, described above, for performing BGA device pin isolation. As shown in FIG. 2, signal trace 24 on PCB 26 is cut and a rework wire 20 is attached to trace 24, close to PCB pad 22, using a high temperature solder. As discussed above, rework wire 20 is not allowed to actually touch PCB pad 22. Rework wire 20 is soldered to trace 24 with a high temperature solder such that the rework wire bond does not reflow when the BGA package reflows.
The above described rework process has several disadvantages which lower the process yield and reduce the reliability of the PCB rework process. In particular, since the rework wire is generally located on the PCB in the BGA pad matrix, the PCB can no longer be screen printed. This is because, on many occasions, the PCB is reworked before any assembly takes place and hence the entire PCB cannot be screen printed due to the rework wire attached to the PCB pad. As a result, each reworked BGA device has to be screen printed individually, leading to loss of time and increased cost of the PCB rework process. The process is further complicated by the need to use solder paste when placing a new BGA device on the PCB. Screen printing a device lowers the volume of solder paste because the solder balls protruding through the stencil lower the deposition volume. Lower paste volumes can cause open circuits due to the volume of solder and flux which make up the solder paste.
Thus, it is desirable to provide a effective and efficient method for performing pin isolation and signal routing for closely spaced pins of a BGA device which also increases the process yield and reliability of the rework process.